Ligand-tuned C-H bond activation/arylation of 2-arylpyridines over pyridine-based N,O/N,N ligated ruthenium-arene complexes

Water-soluble ruthenium(II)-arene complexes [(η⁶-arene)Ru(κ²-L)]ⁿ⁺ (n = 0, 1) ([Ru]-1-[Ru]-10) containing pyridine-based bischelating N,O/N,N donor ligands (L1-L5) are synthesized and employed for the catalytic C-H bond activation/arylation of a wide range of 2-phenylpyridines and aryl halides in water, affording the corresponding mono- and biarylated products. Exploring the reactivity of the synthesized complexes, our investigations, including time-dependent ¹H NMR spectroscopic studies with ruthenium-arene catalysts, demonstrate a remarkable structure-activity relationship for the ligand-tuned C-H activation/arylation of 2-phenylpyridine, where the complexes with bischelating N,O donor-based ligands (acteylpyridine and picolinate) outperform those with N,N donor ligands (iminopyridine). Moreover, among the N,O donor ligands, a distinct effect of the nature of the coordinating oxygen donor on the catalytic activity is also observed, where ruthenium-arene complexes having N,O donor ligands (acetylpyridine) with neutral oxygen-donor atoms exhibit enhanced catalytic activity over those with anionic oxygen-donor atoms (picolinate). The observed trend in the catalytic activity is attributed to the ligand-promoted facile deprotonation and coordination-decoordination interconversion behavior. In addition, molecular structures for a few of the representative complexes ([Ru]-2, [Ru]-4, and [Ru]-5) are authenticated by single-crystal X-ray diffraction studies. (copyright 2018 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)